The present invention relates to a microcomputer which controls an apparatus equipped with a raster scan type cathode ray tube (hereinafter simply called "CRT"), and more particularly to a one-chip microcomputer generating data signals to be used for displaying numerals, letters, symbols, etc. (hereinafter called collectively "characters") on the CRT.
A microcomputer, when employed to control a television receiver, has a variety of functions such as a digital tuning function of receiving a selected broadcasting station wave by means of PLL (Phase Locked Loop) frequency synthesizer technique or voltage synthesizer technique, a volume control function of increasing or decreasing a sound volume and a timer function of setting a time point at which a power switch of the television receiver is turned ON or OFF. For television receivers which display characters such as a selected channel number on the CRT along with video pictures, the microcomputer has a function of generating data signals of characters to be displayed. For that purpose, the microcomputer is provided with a character memory which stores data of each character to be displayed and a video memory which stores address information of the characters in the character memory to make access to the character data to be displayed. In order to display the characters at a desired position on the display screen of the CRT, timing of reading the character data out of the character memory should be in synchronism with vertical and horizontal synchronizing pulses which are used for displaying video pictures. On the other hand, the operation of the microcomputer is under control of a timing signal. Each function is carried out by executing instructions stored in a program memory, and the execution timing and time period of the program is determined by a machine cycle of the microcomputer. Accordingly, the reading timing of the character data which is under control of the execution timing of the program becomes asynchronous with the vertical and horizontal synchronizing pulses of the TV.